Predictive Failure Algorithm For Refrigeration Systems

ABSTRACT

An apparatus and method for predicting failure of a ultralow temperature freezer is disclosed. The freezer includes a variety of temperature sensors, which monitor the temperature of the freezer at various components, such as at the heat exchanger, the condenser, and the evaporator. A controller is in communication with these sensors. The controller monitors these sensors and may determine that the freezer has experienced a performance degradation, or a severe performance degradation. In some embodiments, the controller also monitors other events, such as the actuation of the compressors, and closing of the freezer door. The controller uses temperature information, either absolute sensor readings, or the difference between two different sensor to estimate refrigerant volume and flow rate in the system. In some embodiment, the controller also uses elapsed time from a specific event and a temperature reading to estimate refrigerant volume and flow rate in the system.

This application claims priority of U.S. Provisional Patent Application Ser. No. 61/773,280, filed Mar. 6, 2013, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

There is a need to store biological samples for various purposes. For example, researchers, grad students, and technicians in life science laboratories within universities, medical schools, research hospitals, pharmaceutical companies, biotech companies, and life science contract labs, use these biological samples. Additionally, facilities performing clinical trial work may also may these samples. To do so, ultralow temperature freezers can be used to store samples at temperatures as low as −86° C. In some embodiments, a freezer can store tens of thousands of samples in vials and racks. Furthermore, it is not uncommon for users to have multiple freezers, thereby enabling the storage of many thousands of samples. Due to the ultra low temperatures, these samples can be stored for extended periods of time.

Freezers may have a lifetime of 10 to 15 years. However, after about 5 years, the compressors within the freezer may degrade by about 20%. This degradation may be compensated for by an increase in the operating duty cycle of the compressor. However, this increase in duty cycle cannot continue indefinitely, and eventually the compressor may fail. Failure of a freezer to maintain its ultralow temperature can be disastrous. In some cases, the value of the samples stored in the ultralow temperature freezer may be in the hundreds of thousands of dollars. When a freezer fails to hold temperature, it may sound an alarm or inform the operator in some other way, such as by signaling an audible alert, by SMS text messaging or by sending an email. The operator then attempts to save the samples by moving them, with very limited notice, to a backup freezer if one is available. Since the failure is obviously unscheduled, it is possible that the failure will occur outside of normal working hours. In this case, the operator may be required to commute to the location where the freezer is located. Once there, the operator can begin the process of moving the samples to a backup system.

This scenario has several risks and drawbacks. First of all, the time required to move the samples may be of such a duration that all of the samples cannot be successful moved before they reach an unacceptably high temperature. Secondly, this procedure requires at least one personnel be assigned to emergency duty, in the event of a freezer failure. Third, in order for this emergency process to be successful, the backup refrigeration system must already be at the ultralow temperature. Since the primary freezer failure is not scheduled, this implies that the backup refrigeration system must always be kept at the ultralow temperature, thereby consuming large amounts of energy.

It would be beneficial if there were a system and method for determining in advance, that an ultralow temperature freezer was at risk of failure. In this way, preventative maintenance could be planned in an organized and methodical manner, minimizing the risk to the samples.

SUMMARY

An apparatus and method for predicting failure of an ultralow temperature freezer is disclosed. The freezer includes a variety of temperature sensors, which monitor the temperature of the freezer at various components, such as at the heat exchanger, the condenser, and the evaporator. A controller is in communication with these sensors. The controller monitors these sensors and may determine that the freezer has experienced a performance degradation, or a severe performance degradation. In some embodiments, the controller also monitors other events, such as the actuation of the compressors, and closing of the freezer door. The controller uses temperature information, either absolute sensor readings, or the difference between two different sensor to estimate refrigerant volume and flow rate in the system. In some embodiment, the controller also uses elapsed time from a specific event and a temperature reading to estimate refrigerant volume and flow rate in the system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of an ultralow temperature freezer that may be used according to one embodiment;

FIG. 2 shows a schematic representation of a controller used with the freezer of FIG. 1;

FIG. 3 shows a failure analysis process according to one embodiment;

FIG. 4 shows a failure analysis process according to another embodiment;

FIG. 5 shows a failure analysis process according to another embodiment;

FIG. 6 shows a failure analysis process according to another embodiment;

FIG. 7 shows a failure analysis process according to another embodiment;

FIG. 8 shows a failure analysis process according to another embodiment; and

FIG. 9 shows a failure analysis process according to another embodiment.

DETAILED DESCRIPTION

Embodiments of the present invention allow a controller to monitor the operations of the ultralow freezer system. When the controller detects anomalies or deviations from normal operation, a warning can be created while the freezer can still maintain normal ultralow temperatures.

FIG. 1 shows a schematic view of an ultralow temperature freezer in accordance with one embodiment. The ultralow temperature freezer 10 is made of two single stage refrigeration systems that are cascaded together through the use of a heat exchanger 100.

The first stage, or high side stage, includes a first compressor 20, a first condenser 30, a first filter drier 40, a first capillary tube 50 and a J-shaped suction accumulator 60. Like a traditional single stage refrigeration system, this first stage uses a first compressor 20, which absorbs a first stage refrigerant vapor and adds mechanical energy to it such that it exits the compressor at high temperature and high pressure. This high pressure and high temperature vapor gas is piped into the first condenser 30, where gas heat is removed by air flow through the heat sinks of the first condenser. These heat sinks may be aluminum fins in one example. While this process maintains a constant pressure, the heat content in the first stage refrigerant gas is removed and therefore, the first stage gas is condensed to liquid form inside the first condenser 30. The high pressure liquid is then passed through the first filter drier 40. As its name implies, a filter drier removes moisture and water from the refrigeration system, it also filters any debris that might be in the system. The high pressure first stage refrigerant liquid enters the first capillary tube 50, which functions as an expansion device in the system. The sudden cross section area drop in the first capillary tube 50 significantly reduces the pressure of the first stage refrigerant. As the first stage refrigerant flows through the first capillary tube 50, the friction inside the first capillary tube 50 forces the first stage refrigerant pressure to reduce even further, as well as the temperature. This low pressure first stage refrigerant then enters the cascade heat exchanger 100, which acts as an evaporator for the first stage system. As the low pressure first stage refrigerant flows through the cascade heat exchanger 100, it absorbs the heat from the second stage, which causes the first stage liquid refrigerant to evaporate. From the cascade heat exchanger 100, the first stage refrigerant then goes to a J-type suction accumulator 60, which is responsible for eliminating any excess liquid first stage refrigerant from reaching the first stage compressor during any part of the refrigeration cycle. The first stage refrigerant vapor then returns back to first compressor 20 and the cycle repeats. In some embodiment, the first stage refrigerant may be R404A refrigerant, which provides an evaporator temperature of −35° C. to −40° C.

The first stage refrigerant runs uphill in the insulated cascade heat exchanger 100 to provide a fully flooded condition for the condensing process of the second stage refrigerant, which, in one embodiment, may be R508B.

The second stage may include a second compressor 120, a second condenser 130, a second drier 140, a second capillary tube 150, and an evaporator 170.

The condensed liquid of the second stage refrigerant is expanded through the second stage capillary tube 150 and flows into the evaporator 170. In the evaporator 170, the low pressure second stage refrigerant evaporates and provides cooling effect for the storage space of the freezer as it removes the heat from the chamber. From the evaporator 170, the second stage refrigerant gas travels to the second compressor 120. In the second compressor 120, the second stage refrigerant is compressed and pushed out to a second stage de-superheating circuit that is responsible for removing excess heat energy from the second stage discharge gas before it reaches the oil separator 180. This second stage de-superheating circuit may be a second condenser 130. In the oil separator 180, the oil is filtered out, leaving only high pressure second stage refrigerant gas. This second stage refrigerant flows to the cascade heat exchanger 100 which condenses the second stage refrigerant vapor into liquid and the second stage refrigeration cycle repeats.

While the first stage works just like a traditional single stage system, the second stage may have additional parts such as an oil separator 180, and auto reset pressure switch 190. The oil separator 180 separates out most compressor oil in the discharge line that might have been pushed out by the second compressor 130. A combination of using an oil separator 180 and adding a small amount of R290 (propane) into the second stage system may increase the system's robustness by reducing the probability of oil logging.

While FIG. 1 describes one embodiment that may be used to create an ultralow temperature freezer, it is understood that other embodiments are possible. However, in some embodiments, there may be two stages of cooling, which are cascaded using a heat exchanger. Each stage may be designed similar to a conventional refrigeration system, having a compressor, a condenser, and an evaporator. Additional components may be included based on the desired design parameters. Consequently, the present disclosure is not limited to the embodiment shown in FIG. 1.

Failures in ultralow temperature freezers can be caused by several conditions. For example, flow restrictions of refrigerant in either the first stage or the second stage may cause unacceptable behavior. Failures in the condenser, degraded performance of the compressor, and refrigerant loss are also causes of system failures.

The present system monitors system operation and attempts to predictively determine degraded performance and evaluate the extent of degraded performance, which may indicate a prospective failure. This may be accomplished through the utilization of various sensors distributed in the ultralow temperature freezer 10. The following describes the placement and use of various sensors in the freezer 10. It is understood that not all of these sensors may be used in all embodiments.

In one embodiment, the sensors used are all temperature sensors. In some embodiments, there may be a sample chamber temperature sensor 200, an ambient temperature sensor 210, a condenser temperature sensor 220, an evaporator inlet temperature sensor 230, an evaporator outlet temperature sensor 240, a first stage heat exchanger inlet temperature sensor 250, a first stage heat exchanger outlet temperature sensor 260, a second stage heat exchanger inlet temperature sensor 270 and a second stage heat exchanger outlet temperature sensor 280. In some embodiments, pressure sensors may also be employed.

As seen in FIG. 2, these sensors are in communication with a controller 300, which receives the data from each sensor. These sensors may be analog sensors, in which case, the controller 300 includes one or more analog to digital converters 310 to convert the sensor information into a digital value. In other embodiments, the sensors may output a digital value, which can be used directly by the controller 300. The controller also includes a processing unit 320 in communication with a memory device 330. This memory device 330 contains instructions, which when executed, can perform the predictive failure analysis described herein. These instructions may be written in any suitable programming language, the choice of which is not limited by this disclosure. The controller 300 also includes an alert mechanism 340. In some embodiments, this may include a visual alert, such as one or more colored lights, or a digital display unit. In other embodiments, an audio alert may be used. In yet other embodiments, the alert may compromise a communication to an operator, such as a SMS text message, an email, or an automated telephone call.

Using the controller 300 in conjunction with the various temperature sensors, a variety of conditions can be detected. In addition, other sensors may also be employed. For example, a signal may be generated by the controller 300, which is used to actuate the first compressor 20, such as through use of a relay. A second signal may be generated by the controller 300 to actuate the second compressor 120. The duty cycle of these signals is indicative of the duty cycle of the compressors. Pressure sensors (not shown) may be used to insure that each of the compressors 20, 120 are actually operating. In another embodiment, the average power consumption of each of the compressors 20, 120 may be monitored. If the power consumption of each of the compressors 20, 120 in the operating and non-operating states is known, the duty cycle of each compressor can be determined based on average power consumption. Other methods of determining the duty cycle of the compressors 20, 120 may also be used.

The following describes various processes that can be executed by the controller to predict potential freezer failures.

FIG. 3 shows a first process that can be used to monitor the freezer 10. In this embodiment, the controller monitors the duty cycle of the compressors 20, 120 as described above. First, the controller 300 ensures that the door is not open, as determined by another sensor (not shown), as shown in step 400. This sensor may be a proximity sensor, or some other type that is used to determine whether the door is closed. If the door is open, the controller 300 continues to monitor the door sensor until the door closes. If the door is closed, the controller checks if the chamber is at the desired temperature, using temperature sensor 200, as shown in step 410. Once the freezer 10 has reached the desired temperature, the controller 300 may wait one or more refrigeration cycles, as shown in step 420. A refrigeration cycle is defined as a two step sequence where the compressors 20, 120 are actuated to lower the temperature, and turn off once that temperature is reached. The duty cycle is then checked, as shown in step 430. If the duty cycle of the compressors 20, 120 is determined to be less than a first predetermined value (D1), such as less than 80%, the freezer 10 is assumed to be operating normally, and no problem is reported as shown in step 440. If the duty cycle is greater than the first predetermined value (D1) but less than a second predetermined value (D2), such as between 80-90%, a performance degradation is detected, and a first level of alert may be actuated, as shown in step 450. This first level of alert may comprise a yellow light, or an audio or communication alert. If the duty cycle is determined to be greater than the second predetermined value (D2), such as greater than 90%, a severe performance degradation is detected, and a second level of alert may be actuated, as shown in step 460. This second level of alert may comprise a red light, or an audio or communication alert. The use of two levels of alert gives the operator some visibility into the urgency of the issue and may be instrumental in allowing the operator to properly schedule a preventative maintenance operation. However, in other embodiments, only one level of alert may be employed. The process of FIG. 3 is used to detect degradation in the performance of the compressors 20, 120. The values described for the first and second predetermined values are illustrative, and the actual values used may vary from these numbers. The actual values may be determined empirically, as a result of testing of multiple freezers.

The process of FIG. 4 is used to detect a potential flow restriction in the second stage of freezer 10. The controller first measures the ambient temperature, using sensor 210, to insure that it is within the specified operating range, as shown in step 500. This specified range may be any temperature greater than 15° C., for example. If it is not, no problem is reported as shown in step 550. Similarly, the controller checks to insure that the door is closed, in step 510. If it is not, no problem is reported, as shown in step 550. The controller 300 then checks if both of the compressors 20, 120 are operating, in step 520. This can be determined by checking the signal generated to actuate compressors, by sensing relays located on a power module, or using another method. Additionally, the controller 300 may use pressure sensors to insure that the compressors 20, 120 are operating properly. If the compressors 20, 120 are not both active, no problem is reported as shown in step 550. The controller 300 then measures the temperature of the inlet of the evaporator 170 using sensor 230, as shown in step 530. If the temperature of the evaporator inlet is greater than a first predetermined temperature (T1), such as −100° C., the system is assumed to be properly working and no problem is reported in step 550. However, if the temperature is less than this first predetermined temperature (T1), the controller then checks the heat exchanger 100 to see if its temperature is below a second predetermined temperature (T2), such as −45° C., as shown in step 540. In one embodiment, sensor 270 is used to measure the temperature of the heat exchanger 100. If it is not, then the controller reports no problem, as shown in step 550. However, if the temperature of the heat exchanger is below that second predetermined temperature (T2), the controller identifies a potential problem. The excessively low temperatures of the evaporator 170 and heat exchanger 100 may indicate a flow restriction in the second stage of the freezer, such as perhaps in the second capillary tube 150. The controller 300 determines that a severe performance degradation has occurred and actuates a second level of alarm, as shown in step 560. Again, the values described for the first and second predetermined values are illustrative, and the actual values used may vary from these numbers. The actual values may be determined empirically, as a result of testing of multiple freezers.

The process of FIG. 5 is executed when the freezer 10 is first activated and is used to measure the time required to bring the internal chamber to the desired temperature. First, the controller 300 checks that the ambient temperature is within a specified range, using sensor 210, as shown in step 600. This specified range may be, for example, between 15-32° C. If the ambient temperature is outside the specified range, an ambient temperature alert may be actuated. However, as this is not a predictive failure mode, the controller 300 reports no problem as shown in step 650. If the ambient temperature is acceptable, the controller 300 then determines whether the door is open using another sensor (not shown). If the door is open, the controller 300 reports no problem as shown in step 650. However, a door ajar alarm may be actuated. If the door is closed, the controller then measures the time required for the internal chamber to reach the desired temperature. The temperature of the internal chamber is measured using sensor 200. If the time required to reach the desired temperature is less than a predetermined value, the controller 300 reports no problem as shown in step 650. However, if the time required to reach the desired temperature is greater than a predetermined value (Δt), the controller 300 reports a performance degradation, as shown in step 640. This predetermined value may be between 300 and 800 minutes, or, in one embodiment, may be about 8 hours. A first level of alert may be actuated in this case. In another embodiment, the time required to reach the desired temperature is compared against a larger second predetermined value, such as 10 hours. If this time is exceeded, the controller 300 may report a severe performance degradation and a second level of alert may be activated. The failure to reach temperature in a predetermined amount of time may indicate an unbalanced refrigerant charge, caused by a refrigerant loss in either of both refrigeration stages. The predetermined values used above are illustrative and may vary based on the actual design. As such, the disclosure is not limited to any particular values.

FIG. 6 monitors the rate at which the heat exchanger 100 reaches temperature. The time required for the heat exchanger 100 to reach a predetermined temperature may be indicative of an abnormal cooling rate from the first stage, possibly caused by first stage refrigerant loss, inefficient operation of the first condenser 30, or loss of efficiency in the first compressor 20. The controller 300 first waits until the first compressor 20 turns on, as shown in step 700. Once the first compressor 20 is on, the controller measured the time required by the heat exchanger 100 to reach a predetermined temperature, as shown in step 710. Sensor 250 may be used to measure the temperature of the heat exchanger 100. The controller 300 then compares this time to several predetermined values in step 720. If the time is less than a first predetermined value (T1), such as between 4 and 15 minutes, the system is operating correctly and no problem is reported, as shown in step 730. If the time required is greater than the first predetermined time (T1), but less than a second predetermined time (T2), such as between 6 and 30 minutes, a performance degradation is reported, as shown in step 740, and a first level of alert may be actuated. If the time required is greater than the second predetermined time (T2), the controller 300 reports a severe performance degradation, as shown in step 750, and a second level of alert may be actuated. The values described for the first and second predetermined values are illustrative, and the actual values used may vary from these numbers. These values are illustrative and the actual values may be determined empirically as a result of testing of multiple freezers.

FIG. 7 shows a process that may be used to verify the operation of the condenser 30. The controller 300 determines the difference between the ambient temperature, as measured by sensor 210, and the temperature of the condenser 30, as measured by sensor 220, as shown in step 800. The controller then determines an appropriate action based on this temperature difference, as shown in step 810. For example, if the temperature difference is less than a first predetermined value (ΔT1), which may be between 5° C. and 10° C., or more particularly, 8° C., the controller reports there is no problem, as shown in step 820. If the temperature difference is greater than the first predetermined value (ΔT1), the controller 300 may report a performance degradation, as shown in step 830, and activate a first level of alert. In some embodiments, if the temperature difference is greater than a second predetermined value (ΔT2), which may be between 11° C. and 20° C., such as 15° C., the controller 300 may report a severe performance degradation, as shown in step 840, and activate a second level of alert. The values described for the first and second predetermined values are illustrative, and the actual values used may vary from these numbers. The actual values may be determined empirically, as a result of testing of multiple freezers.

The inability for the condenser to cool, as evidenced by a large temperature difference between it and the ambient, may be indicative of a clogged condenser filter, dirt on the condenser 30, or a blockage or obstruction between the condenser vent and the external environment.

FIG. 8 shows another process flow, which is designed to detect possible first stage malfunctions, such as flow restrictions or refrigerant loss. In this embodiment, the controller 300 waits for the first compressor 20 to be operational, as determined by, for example, relays in the power module, as shown in step 900. Once the first compressor 20 is active, the controller 300 waits for the heat exchanger 100 to reach a temperature of less than a first predetermined value (T1), such as −35° C., as shown in step 910. The temperature of the heat exchanger 100 may be measured using sensor 250. The controller 300 then waits for the second compressor 120 to be operational, as determined by a relay in the power module, as shown in step 920. Once the second compressor 120 is active, the controller 300 monitors the temperature of the heat exchanger 100 using sensor 270, as shown in step 930. The controller 300 compares the temperature of the heat exchanger 100 to one or more predetermined values, as shown in step 940. In some embodiments, the controller waits one or more refrigeration cycles before making this comparison. If the temperature of the heat exchanger 100 remains below a first predetermined value (T1), such as −25° C., the controller reports no problem, as shown in step 950. If the temperature of the heat exchanger raises above the first predetermined value, but remains below a second predetermined value (T2), such as −10° C., the controller 300 may report a performance degradation, as shown in step 960 and actuate a first level of alert. If the temperature of the heat exchanger 100 rises above the second predetermined value, the controller reports a severe performance degradation, as shown in step 970, and actuates a second level of alert. In some embodiment, two predetermined values are not used. In this case, one of steps 960 or 970 is not included.

It should be noted that any or all of the processes shown in FIG. 3 through 8 may be executed by the controller 300.

FIG. 9 shows another failure analysis process. By disposing temperature sensors 230, 240 on the two ends of the evaporator, the controller 300 can monitor the temperature difference between the inlet to the evaporator 170 and the outlet from the evaporator 170, as shown in step 1000. If this difference differs from this nominal value by less than a first predetermined value (ΔT1), such as 10° C., the controller 300 reports no problem, as shown in step 1020. If the temperature difference differs from this nominal value by more than the first predetermined value (ΔT1), but below a second predetermined value (ΔT2), such as 20° C., the controller may indicate a performance degradation, as shown in step 1030, and may actuate a first level of alert. If the temperature difference differs from this nominal value by more than the second predetermined value (ΔT2), it may signify that there is a flow restriction in the second stage. In this case, the controller may indicate a severe performance degradation, as shown in step 1040 and actuate a second level of alert. In some embodiments, if this difference is too small, it may indicate an inability to remove heat from the internal chamber, which may be a second stage refrigerant imbalance or loss.

In addition, the controller 300 may execute other processes. Similarly, the disposition of sensors 250, 260, 270, 280 at the various inlets and outlets of the heat exchanger 100 allows various measurements to be made. For example, the temperature difference between the second stage inlet and second stage outlet can be measured. Similarly, the temperature difference between the first stage inlet and the first stage outlet can be measured. These temperature differences may be indicative of a flow restriction in the first or second stage, or may indicative a loss of first stage refrigerant or second stage refrigerant. Furthermore, the difference between the temperature of the first stage of the heat exchanger 100, as measured using sensors 250,260, and the temperature of the second stage of the heat exchanger 100, as measured using sensors 270, 280, may provide information regarding the efficiency of heat exchange occurring within the heat exchanger 100.

Thus, in some embodiments, the controller 300 monitors the temperatures at various locations within the freezer. These temperatures are used to estimate the flow rate or heat capacity of the refrigerant running through the first and/or second refrigeration stage. Thus, in some embodiments, the controller 300 may use absolute temperature values to estimate the volume of the refrigerant and/or the flow rate through the system. In other embodiments, the controller 300 may use the difference in temperature between two points in the freezer to estimate the volume of the refrigerant and/or the flow rate through the system. In another embodiment, the controller 300 may use the elapsed time for the temperature at a certain point to reach an expected value following a specific event to estimate the volume of the refrigerant and/or the flow rate through the system.

Additionally, other temperature checks may also be performed. For example, as described above, if the ambient temperature is too high, as measured by sensor 210, a warning or alert may be generated. In addition, if the internal chamber deviates from the desired temperature by more than a predetermined amount, a warning or alert may be generated.

The alerts described herein may be generic alerts. In other words, all first level alerts may result in a single yellow light be actuated. However, in other embodiments, the operator has the ability to determine which of the processes described herein generated the alert. This may be through, for example, a digital display, which assigns a different error code to each fault. IN other embodiments, a different mechanism is used to provide the operator with more detailed fault information. In addition, in some embodiments, the operator may be able to view the values of the various temperature sensors in the freezer 10, using, for example, a digital display.

Additionally, the disclosure describes the use of various temperature sensors throughout the freezer 10. The disclosure is not limited to this embodiment. For example, in some embodiments, there may be fewer sensors. For example, there may be only one sensor associated with the heat exchanger 100. In one embodiment, there may be only one sensor associated with the evaporator 170. The number and locations of the sensors may impact the ability to perform one or more of the fault analysis processes described herein. However, the other failure analysis processes may still be executed.

The processes described herein may be executed repeatedly and periodically. For example, in one embodiment, the various sensors in the freezer 10 may be read by the controller 300 every 250 milliseconds. In some embodiments, the controller 300 executes the processes described herein every 2-10 seconds, although other frequencies are within the scope of the disclosure.

The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein. 

What is claimed is:
 1. A method of predicting failure of an ultralow temperature freezer, comprising: monitoring a duty cycle of a compressor used in said freezer; comparing said monitored duty cycle to a first predetermined value to determine if said freezer is operating properly; and comparing said monitored duty cycle to a second predetermined value to determine if said freezer has experienced a severe performance degradation; and alerting an operator if said freezer has experienced a severe performance degradation.
 2. The method of claim 1, further comprising determining said freezer has experienced a performance degradation if said monitored duty cycle between said first predetermined value and said second predetermined value.
 3. The method of claim 2, wherein said alerting step comprising actuating a different alert if a severe performance degradation is determined to exist.
 4. A method of predicting failure of an ultralow temperature freezer, wherein said freezer comprises two cascaded refrigeration stages, an evaporator disposed in a storage chamber of said freezer to remove heat from said storage chamber, and a heat exchanger disposed between said two cascaded stages to remove heat from a second refrigeration stage, comprising: monitoring a temperature of said evaporator; comparing said monitored temperature of said evaporator to a first predetermined value; monitoring a temperature of said heat exchanger; comparing said monitored temperature of said heat exchanger to a second predetermined value; determining said freezer has experienced a severe performance degradation if said monitored temperature of said evaporator is less than said first predetermined value and said monitored temperature of said heat exchanger is less than said second predetermined value; and alerting an operator of said severe degradation.
 5. A method of predicting failure of an ultralow temperature freezer, comprising: determining a door of said freezer has been closed; measuring elapsed time from said determination until a storage chamber of said freezer reaches a predetermined temperature; alerting an operator of a performance degradation if said elapsed time is greater than a first predetermined value.
 6. The method of claim 5, further comprising alerting an operator of a severe performance degradation if said elapsed time is greater than a second predetermined value, greater than the first predetermined value.
 7. The method of claim 5, wherein said measuring and alerting steps are only performed if an ambient temperature is within a predetermined range.
 8. A method of predicting failure of an ultralow temperature freezer, wherein said freezer comprises two cascaded refrigeration stages, a first compressor disposed in a first refrigeration stage and a heat exchanger disposed between said two cascaded stages to remove heat from a second refrigeration stage, comprising: determining said first compressor is actuated; monitoring an elapsed time from said actuation until said heat exchanger reaches a predetermined temperature; and alerting an operator of a performance degradation if said elapsed time is greater than a first predetermined value.
 9. The method of claim 8, further comprising: alerting an operator of a severe performance degradation if said elapsed time is greater than a second predetermined value, greater than said first predetermined value.
 10. A method of predicting failure of an ultralow temperature freezer, comprising: monitoring a temperature of a compressor used in said freezer; monitoring an ambient temperature outside said freezer; comparing said monitored temperature of said compressor to said ambient temperature; alerting an operator of a performance degradation if a difference between said monitored temperature of said compressor and said ambient temperature is greater than a first predetermined value.
 11. The method of claim 10, further comprising: alerting an operator of a severe performance degradation if a difference between said monitored temperature of said compressor and said ambient temperature is greater than a second predetermined value, greater than said first predetermined value. 